The present invention relates to seals in a gas turbine for supplementing the chordal hinge seals between turbine nozzles and a turbine nozzle support ring and particularly relates to supplementary seals for substantially minimizing or eliminating leakage losses past the chordal hinge seals.
In a gas turbine, hot gases of combustion flow from combustors through first-stage nozzles and buckets and through the nozzles and buckets of follow-on turbine stages. The first-stage nozzles typically include an annular array or assemblage of cast nozzle segments each containing one or more nozzle stator vanes per segment. Each first-stage nozzle segment also includes inner and outer band portions spaced radially from one another. Upon assembly of the nozzle segments, the stator vanes are circumferentially spaced from one another to form an annular array thereof between annular inner and outer bands. A nozzle retaining ring coupled to the outer band of the first-stage nozzles supports the first-stage nozzles in the gas flow path of the turbine. An annular nozzle support ring, preferably split at a horizontal midline, is engaged by the inner band and supports the first-stage nozzles against axial movement.
In an exemplary arrangement, eighteen cast segments are provided with two vanes per segment. The annular array of segments are sealed one to the other along adjoining circumferential edges by side seals. The side seals seal between a high pressure region radially inwardly of the inner band, i.e., compressor discharge air at high pressure, and the hot gases of combustion in the hot gas flow path which are at a lower pressure.
Chordal hinge seals are used to seal between the inner band of the first-stage nozzles and an axially facing surface of the nozzle support ring. Each chordal hinge seal includes an axial projection which extends linearly along a chord line of the inner band portion of each nozzle segment. Particularly, the chordal hinge seal extends along an inner rail of each segment and which rail extends radially inwardly of the inner band portion. The chordal hinge seal projection lies in sealing engagement with the axially opposite facing sealing surface of the nozzle support ring.
During operation and/or repair of the first-stage nozzle, it has been found that warpage can leave gaps between the chordal hinge seals and the sealing surface of the nozzle support ring. These gaps enable leakage past the chordal hinge seals from the high pressure area radially within the annular inner band into the hot gas flow path. That is, the chordal hinge seals are inadequate to prevent leakage flow as the chordal hinge seal projections lose contact with the sealing surface of the nozzle support ring. Consequently, there is a need for a supplemental seal at the interface of the first-stage nozzles and nozzle support ring to minimize or eliminate the leakage flow past the chordal hinge seals.
In accordance with a preferred embodiment of the present invention, there is provided a supplemental seal between the first-stage nozzles and the nozzle support ring which eliminates or minimizes leakage past the chordal hinge seals. The supplemental seal includes a seal body which extends in an arcuate cavity in one of the axially opposed sealing surfaces of the nozzle support ring and nozzle segment radially outwardly of the chordal hinge seal. The seal body has preferably a first, generally U-shaped portion in cross-section, and a pair of reversely extending, generally U-shaped marginal portions in cross-section along opposite sides of the U-shaped portion. With the seal body disposed in the cavity and at turbine operating conditions, the marginal portions of the seal body lie in sealing engagement against an interior surface of the cavity, e.g., the base of the cavity, and the opposite opposed axially sealing surface whereby any leakage flow past the chordal hinge seal from the high pressure region to the low pressure region of the hot gas path is substantially eliminated.
In a particularly preferred form of the present invention, the supplemental seal is formed of sheet metal, preferably a pair of sheet metal plates secured, for example, by welding, to one another and bent into the aforementioned cross-sectional configuration. To install the supplemental seal, the seal is first placed in a compressed state and maintained in that compressed state during installation. To accomplish this, the seal may be wrapped by a material which, at turbine operating or near-operating conditions such as temperature, disintegrates, releasing the seal to expand in the cavity which, under preload, biases the marginal portions of the seal body against the sealing surfaces. The wrap may be formed of a Kevlar(copyright) 29 or may be formed of a high-strength plastic material, such as Lexan(trademark) or Ultem(trademark) clips to hold the seal in a compressed condition during installation. Alternatively, epoxy may be applied to the compressed seal to maintain the seal in the cavity in the compressed condition, the epoxy releasing the seal body at operating or near-operating turbine conditions for sealing engagement with the opposed sealing surfaces.
In a preferred embodiment according to the present invention, there is provided a gas turbine comprising a turbine nozzle support ring having a generally axially facing first surface, a turbine nozzle segment having at least one stator vane and including an inner band having a second surface in axial opposition to the first surface, one of the first and second surfaces defining a cavity opening generally axially toward another of the first and second surfaces and a flexible seal in the cavity including a seal body having a first, generally U-shaped portion in cross-section and a pair of reversely extending, generally U-shaped marginal portions in cross-section along opposite sides of the U-shaped portion, the marginal portions in sealing engagement with an interior surface of the cavity of one surface and another of the first and second surfaces, respectively.
In a further preferred embodiment according to the present invention, there is provided a turbine comprising a turbine nozzle support ring having a generally axially facing first surface, a plurality of turbine nozzle segments having an annular array of stator vanes and an annular second surface in axial opposition to the first surface, each of the segments including an axial extending projection for sealing engagement with the first surface forming a first seal therewith, one of the first and second surfaces having a cavity opening generally axially toward another of the first and second surfaces and at a location radially outwardly of the first seal and a flexible seal in the cavity including a seal body having a first, generally U-shaped portion in cross-section and a pair of reversely extending, generally U-shaped marginal portions in cross-section along opposite sides of the U-shaped portion, the marginal portions in sealing engagement with an interior surface of the cavity of one surface and another of the first and second surfaces, respectively.